The present invention relates to new phosphine ligands, to their preparation and to their use in catalytic reactions, especially organic coupling reactions employing aryl, heteroaryl or vinyl halides and pseudohalides as educts.
Organic coupling reactions are an important tool to form carbon-carbon and carbon-heteroatom bonds. The popularity of coupling reactions is partly due to their tolerance against the presence of functional groups. This characteristic allows the use of coupling reactions in the synthesis of very complex molecules and thus, coupling reactions are widely used in the chemical and pharmaceutical industry, e.g. for the preparation of agricultural chemicals, pharmaceuticals, and dyestuffs, and, if vinyl compounds are coupled, to prepare monomers for polymerization reactions.
Suitable reactants for the coupling reactions are aryl, heteroaryl and vinyl halides, triflates, and other pseudohalides. The coupling reactions are catalyzed by transition metal compounds, typically palladium or nickel compounds. Palladium catalysts are generally advantageous in terms of the breadth of applicability of coupling substrates and in some cases the catalyst activity, while nickel catalysts have advantages in the area of the conversion of chloroaromatics and vinyl chlorides and the price of the metal. Palladium and nickel catalysts used to activate the aryl, heteroaryl and vinyl halides/pseudohalides are palladium(II) and/or nickel(II) as well as palladium(0) and/or nickel(0) complexes, although it is known that palladium(0)/nickel(0) compounds are the actual reaction catalysts. In particular, according to literature sources, coordinatively unsaturated 14-electron and 16-electron palladium(0)/nickel(0) complexes stabilized with donor ligands such as phosphines are formulated as active species.
Amongst the above-mentioned educts for coupling reactions, the iodides are the most reactive ones. It is even possible to use palladium or nickel compounds that are not stabilized by a phosphine or a similar donor ligand when iodides are employed as educts in coupling reactions. However, aryl and vinyl iodides are very expensive starting compounds and moreover produce stoichiometric amounts of iodine salt waste. The remaining educts, i.e. the aryl, heteroaryl and vinyl bromides, chlorides, triflates and other pseudohalides require the use of stabilizing and activating ligands in order to become effective in catalytic production.
Until some years ago, exclusively iodides, bromides, and triflates were used as educts in most of the catalyzed coupling reactions described. Obviously, organic chlorides were not employed as reactants although they should be the most appropriate reactants due to their low costs and great variety. Unfortunately, the chlorides proved to be generally not reactive under the reaction conditions used for coupling of iodides, bromides, and triflates. The low reactivity of chlorides is usually attributed to the strength of the C—Cl bond. Accordingly, the oxidative addition of the chlorides to the metal center of the catalyst (e.g. Pd0) occurs only reluctantly; however, this is the crucial first step in metal-catalyzed coupling reactions. Only within the last years, some progress was made concerning the development of new palladium-based catalysts that are effective in the coupling of chlorides.
The catalyst systems described for coupling reactions often have satisfactory catalytic turnover numbers (TONs) only with uneconomic starting materials such as iodides and activated bromides. Otherwise, in the case of deactivated bromides and especially in the case of chlorides, it is generally necessary to add large amounts of catalyst, usually more than 1 mol %, to achieve industrially useful yields (>90%). In addition, because of the complexity of the reaction mixtures, simple catalyst recycling is not possible, so the recycling of the catalyst also incurs high costs, which are normally an obstacle to realization on the industrial scale. Furthermore, particularly in the preparation of active substances or active substance precursors, it is undesirable to work with large amounts of catalyst because of the catalyst residues left behind in the product. More recent active catalyst systems are based on cyclopalladized phosphines (W. A. Herrmann, C. Brossmer, K. Öfele, C.-P. Reisinger, T. Priermeier, M. Beller, H. Fischer, Angew. Chem. 1995, 107, 1989; Angew. Chem. Int. Ed. Engl. 1995, 34, 1844) or mixtures of bulky arylphosphines (J. P. Wolfe, S. L. Buchwald, Angew. Chem. 1999, 111, 2570; Angew. Chem. Int. Ed. Engl. 1999, 38, 2413) or tri-tert.-butylphosphine (A. F. Littke, G. C. Fu, Angew. Chem. 1998, 110, 3586; Angew. Chem. Int. Ed. Engl. 1998, 37, 3387) with palladium salts or palladium complexes.
However, even with these catalysts, cost-effective chlorides cannot generally be activated satisfactorily from the industrial point of view. Therefore, to achieve high yields, it is necessary to use comparatively large and hence very expensive amounts of catalyst. Unfortunately, the current noble metal prices are still high, so there is clearly a need for improving catalyst productivity. Therefore, despite all the catalyst developments in recent years, only a few industrially applicable reactions have so far been disclosed for the coupling of chlorides.
The properties of transition metal catalyst complexes are recognized to be influenced by both the characteristics of the metal and those of the ligands associated with the metal atom. For example, structural features of the ligands can influence reaction rate, regioselectivity, and stereoselectivity.
Trialkylphosphines with bulky substituents are highly useful ligands for transition metal complexes, especially palladium complexes, as catalysts in various types of coupling reactions. The main reasons for the favorable catalytic properties of trialkylphosphine palladium complexes are the electron-richness and the steric bulk of trialkylphosphine ligands, which favor the formation of low coordinate and highly active Pd complexes also observed with N-heterocyclic carbenes as Pd ligands in cross-coupling reactions. Prominent examples of phosphines are PCy3, P(tert.-Bu)3 and ligands of the Ad2PR type (Ad=1-adamantyl, R═CH2Ph, n-Bu) (Beller et al., Angew. Chem. Int. Ed. 2000, 4153, and WO-A-02/10178). Especially PtBu3 is highly useful; its utility for a wide range of different coupling reactions has been established.
A significant disadvantage of Pd catalysts based on bulky trialkylphosphines, primarily (tert.-Bu)3P, is the lack of flexibility in the design of ligands and catalysts. Detailed structural and electronic modifications (“catalyst fine tuning”) are difficult to realize and this could be the reason why in cross-coupling chemistry this class of ligands was “leader of the pack” only about five years ago. Today numerous other specialized and more powerful catalysts, often based on phosphines and N-heterocyclic carbenes as ligands for Pd are available. Examples of phosphines having a highly variable ligand backbone are the Buchwald type biphenyl based phosphines (S. Buchwald et al., J. Am. Chem. Soc. 1998, 9722, EP-A-1 097 158) and N-phenyl-2-pyrrole based phosphines (M. Beller et al., Chem. Comm. 2004, 38). These types of ligands exhibit a good performance in numerous coupling reactions because they allow a fine tuning of their steric and electronic properties.
One object of the present invention is to provide new phosphines preferably exhibiting crucial properties for good ligands such as electron-richness and efficient-donation as perfectly met in trialkylphosphines, but lacking the disadvantages of the trialkylphosphines, i.e. they should have a variable ligand backbone. The new phosphines should be useful as ligands in new catalyst systems that possess greater substrate flexibility, e.g., the ability to utilize cost-effective organic chlorides as educts, and are suitable for a great variety of industrial scale reactions, preferably coupling reactions, that produce the desired products in high yield, with high catalytic productivity, and/or with high purity.